1. Field of the Invention
The present invention relates generally to a communication system, and in particular, to a method and system for allocating resources by controlling a Transmission Time Interval (TTI) in a communication system.
2. Description of the Related Art
Technologies for providing users with data services in current wireless communication environments are classified into 2.5th or 3rd generation cellular mobile communication technologies such as Code Division Multiple Access 2000 1x Evolution Data Optimized (CDMA2000 1xEVDO), General Packet Radio Services (GPRS) and Universal Mobile Telecommunication Service (UMTS) and Wireless Local Area Network (WLAN) technologies such as an Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless LAN and the like.
Given the development of mobile communication technologies, various LAN wireless access technologies such as an IEEE 802.16 based WLAN and the like are appearing. The LAN wireless access technologies are being provided as an alternative plan for providing a high-speed data service in a wireless environment by replacing a wired communication network such as a cable modem or Digital Subscriber Line (xDSL) with the WLAN in one of a home network environment and a hot spot area such as a public place or school.
However, when the above-described WLAN provides the high-speed data service, there are limitations in providing users with public network services due to propagation interference as well as very limited mobility and limited service coverage.
Efforts at various stages are being made for overcoming these limitations. For example, a large amount of research is being conducted on a mobile Internet technology to compensate for the weak points of the cellular mobile communication system and the WLAN. A mobile Internet system can provide a high-speed data service using various types of Mobile Stations (MSs) in indoor/outdoor stationary environments and mobile environments at pedestrian rate of speed and medium/low speed (of about 60 Km/h).
Alternatively, research is also being conducted on wireless access technologies of future Long-Term Evolution (LTE) in the 3rd Generation Partnership Project (3GPP) system. As an example of the wireless access technologies, schemes are provided for efficiently controlling a downlink Transmission Time Interval (TTI) in a communication system based on Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA).
The conventional system provides a concept of a fixed TTI, whereas the 3G LTE provides only a concept of an adaptive TTI for Quality of Service (QoS) guarantee and control signal reduction.
Next the fixed and adaptive TTI concepts are described below with reference to a 3GPP High-Speed Downlink Packet Access (HSDPA) communication system corresponding to a Code Division Multiple Access (CDMA) system and an OFDM/OFDMA communication system.
FIG. 1 illustrates an information exchange procedure using channels in a conventional HSDPA communication system. In particular, FIG. 1 illustrates a TTI in the 3GPP HSDPA system. For convenience of explanation, the HSDPA system defines a TTI of 2 ms.
Referring to FIG. 1, the HSDPA system configures a Medium Access Control (MAC) Packet Data Unit (PDU) in a period of 2 ms. Hybrid Automatic Repeat reQuest (HARQ) signaling information and Channel Quality Information (CQI) are fed back in every MAC PDU.
A TTI control scheme of the above-described HSDPA system has a structure unsuitable for a low latency requirement in the 3G LTE. That is, the 3G LTE requires Radio Access Network (RAN) Round Trip Time (RTT) latency of less than 10 ms. The 3G LTE provides a minimum TTI unit of 0.5 ms. The TTI and latency provided in the 3G LTE can be expressed as shown in Table 1 and Equation (1) below.
TABLE 1RANlatency(RTT)≈2×(6.5TTI+0.5)ms<10 ms  Equation (1)
The OFDM/OFDMA communication system does not use the above-described TTI concept. In the OFDM/OFDMA communication system, the TTI concept can be compared with a MAC frame configuration concept. The OFDM/OFDMA communication system has a frame unit of 5 ms, such that the low latency requirement of the 3G LTE is not reflected. In the OFDM/OFDMA communication system, a user service characteristic is not reflected due to a fixed frame length.
For this reason, the throughput of the OFDM/OFDMA communication system may be reduced due to a frame overhead, for example, MAP signaling. There is a problem in that it is difficult to reflect a channel characteristic based on fast fading because of the fixed frame length. In this case, a shorter frame length is required.
The adaptive TTI for QoS guarantee and control signal reduction are described herein. FIG. 2 illustrates an adaptive TTI concept defined in the conventional communication system. Specifically, FIG. 2 illustrates radio frame and TTI configurations defined in the conventional 3GPP LTE. The 3GPP LTE only provides the adaptive TTI concept, but it does not provide a concrete scheme thereof.
The problems in a fixed TTI and a fixed frame structure are described herein.
As described above, the 3GPP HSDPA communication system has the fixed TTI of 2 ms and the OFDM/OFDMA communication system has the fixed frame structure of 5 ms. In this case, there is a problem in that a service characteristic of user traffic may not be reflected in the communication system. For example, when burst traffic is simultaneously transmitted, overhead of a MAC header may occur. Moreover, there is a problem in that the communication system may not reflect a low latency requirement of the 3GPP LTE and is unsuitable for rapid variation in a channel environment. When the channel environment is good, there is a problem in that an unnecessarily short TTI may cause overhead of a MAC PDU header in the 3GPP HSDPA communication system, MAP overhead in the OFDM/OFDMA communication system, and uplink signaling overhead.
In comparison with the CDMA communication system, the above-described OFDM/OFDMA communication system has more flexibility in resource allocation of a symbol/subcarrier unit. However, the OFDM/OFDMA communication system may have the above-described problems due to the fixed frame structure.
In the 3GPP LTE, a basic TTI of 0.5 ms is considered and the necessity for an adaptive TTI that is capable of realizing QoS desired by users is only proposed. A concrete control scheme is not provided in the 3GPP LTE.
Thus, there exists a need for a scheme for optimizing and controlling a TTI and allocating resources according to channel states and service characteristics of user traffic in the case where a channel state and traffic requiring low latency rapidly vary, in the case where low latency is not required and a channel state and burst traffic do not rapidly vary, and in the case where rapid adaptation to a channel state is not required.